Know Sound Design: What's that Jack for?

Nearly every modern analog monosynth includes a jack on the back panel called “Ext In” and whenever the topic comes up in my workshops and classes, students often ask, “What’s it for?”

As it turns out, the External Input is arguably one of the most powerful features on an analog synth, because it allows you to route any audio signal into the synth’s filter and amplifier engines and use it in place of (or in addition to) the onboard oscillators. This month, we’ll take a look at two useful techniques for making the most of this often overlooked feature.

Method 1: Many keyboardists think the ability to mix and match filters and oscillators is only available in modular rigs, but thanks to the external input, that’s definitely not the case—especially if you have your DAW controlling your hardware synths via MIDI. If so, combining two synths in your sequencing software is easy, just copy the same sequence to two different tracks—each routed to a different synth—with one of those synths including an external input for its synthesis engine.

For example, the Arturia MiniBrute includes an external input that has its own volume fader on the synth’s mixer. As a result, you can use any other synth in your rig as the oscillator bank. Just set the source synth’s filter cutoff to max and use a simple gated amplifier envelope (with extended release time, if appropriate). Then plug the output of that synth into the external input of your “processing” synth (in this case, the MiniBrute) and you’re in business. From there, send the same sequence to both synths, tweak the filter, its envelope, and the amp envelope of the MiniBrute, and voila, you’ve got a hybrid synth without the fuss of a modular rig. (Note that this will also work in a live context by sending MIDI data from your controller, when set to the same channel of your paired synths.)

Method 2: The external input is also a great way to warm up your softsynths. Both Ableton Live and Apple Logic include software effects modules that can route signals from a free output on your audio interface to your external processing synth, then return the audio from the synth’s output to a second free input on the interface. Logic’s module is called I/O (you’ll find it in the Utilities effect menu) whereas Ableton’s is called External Audio Effect. In either case, just place one of these devices after your softsynth, with its oscillators, filter, and amp envelope set up as described above, and route the same MIDI data to both the softsynth and processing synth. If you’ve followed the steps correctly, you’ll now have real analog filters and VCAs processing the tone generators of your softsynth, which can really warm up the sound of digital sources.

In this month’s web-audio clips, I included examples of the Arturia MiniBrute filtering a Moog Little Phatty, then the Moog filters applied to the Arturia oscillators, and finally Ableton’s Operator being filtered by the Moog. All three have distinctly different sounds, each with its own character. So check the back panels of your hardware synths and you may well be ready to roll with this handy trick.